1. Field of the Invention
The present invention relates to a method for joining a laser transmitting resin member and a porous member, a method for joining thermoplastic resin, and a fuel cell. The invention specifically relates to a joining technique in which peel strength of a joined body which is to be obtained is improved, a joining technique which realizes ease of material handling, reduction in the manufacturing cost and an excellent laminated assembly so as to realize satisfactory airtightness of a cell, and a technique for producing fuel cells that miniaturizes the fuel cells and realizes excellent resistivity of a proton exchange membrane (hereinafter, simply referred to as “PEM”).
2. Description of the Related Art
The techniques of joining laser transmitting resin members and porous members have various uses, and this technique can be applied to, for example, membrane electrode joined bodies and cell structures of the fuel cells. Conventionally, such techniques are applied to the joining of different kinds of synthetic resin materials, and when two different kinds of synthetic resin materials are overlapped so as to be joined, one of the overlapped synthetic resin materials is made to be transmissive with respect to laser beams, and the other one is made to be absorptive with respect to laser beams. After a metal mesh whose melting point is higher than that of the synthetic resin materials is made to intervene on a joined surface of the synthetic resin materials, a laser beam is emitted from a direction of the laser transmitting synthetic resin material. This method for joining different kinds of synthetic resin materials has been proposed (see Japanese Patent Application Laid-Open No. 60-214929 (Patent Document 1)).
In this joining technique, when non-compatible laser absorptive resin and laser transmitting resin are joined, the metal mesh whose melting point is higher than these resins is made to intervene between the two kinds of the resins, and the two resins are entwined with the metal mesh so that the resins are joined to the metal mesh. As a result, the resins are mechanically joined.
In general, in a fuel cell, an electric cell, in which a separator is laminated on both sides of a planar membrane electrode assembly (hereinafter, simply referred to as “MEA”), compose one unit, and a plurality of electric cells are laminated so that a fuel cell stack is formed. The MEA has a three-layered structure in which an electrolyte membrane made of ion exchange resin or the like is held between a pair of gas diffusion electrodes forming a cathode and an anode. The gas diffusion electrodes are constituted so that a gas diffusion layer is formed on the outside of an electrode catalyst layer in contact with the electrolytic membrane. The separator is laminated so as to contact with the gas diffusion electrodes of the MEA, and a gas flow channel for circulating gas and a coolant flow channel are formed between the separator and the gas diffusion electrodes. According to such a fuel cell, for example, hydrogen gas as fuel is allowed to flow in the gas flow channel which faces the gas diffusion electrode on the anode side, and oxidizing gas such as oxygen or air is allowed to flow in the gas flow channel which faces the gas diffusion electrode on the cathode side. As a result, the electrochemical reaction is produced, and electricity is generated.
Since the fuel cells have the above structure, when the each units are laminated, the separators of the adjacent electric cells are overlapped. The laminated form is more concretely explained. The separators are constituted so that a resin layer is formed via a joint portion on the outer periphery of a metallic member, and the resin layers of the adjacent separators are overlapped. The joining technique for the resin layers is, therefore, applied to the joining technique for the separators.
The technique for joining the resin layers to be applied to the joining of the separators of the fuel cells includes the following. That is, when different kinds of synthetic resin materials are overlapped so as to be joined, one of the two different kinds of the synthetic resin materials is made to be not absorptive with respect to a laser and the other one is made to be absorptive with respect to a laser. After the two synthetic resin materials are overlapped, a laser is emitted from the direction of the nonabsorptive synthetic resin material, so that the synthetic resin materials are easily joined without reducing the strength of both the synthetic resin materials. Such a technique is disclosed in Japanese Patent Application Publication No. 62-49850 (see Patent Document 2). Moreover, one of the overlapped resin materials is a resin material that uses laser nonabsorptive coloring matter, and the other one is a resin material that uses laser adsorptive coloring matter. A technique for joining the resin materials is proposed in Japanese Patent Application Laid-Open No. 2001-71384 (Patent Document 3). Further, when the laser transmitting resins are joined, solid inorganic matter is allowed to intervene at their interface, a laser is aimed at the solid inorganic matter so that the solid inorganic matter is heated and both the resins are joined. This technique is disclosed in Japanese Patent Application Laid-Open No. 2001-232687 (see Patent Document 4).
Another technique relating to the method for joining separators of a fuel cell is explained below. The viscosity of the sealing agent at the time of application is set to 1000 to 9000 Pa·s, and application pressure, application speed, treatment temperature and treatment time are made to be appropriate so that a seal is formed. This technique is disclosed in Japanese Patent Application Laid-Open No. 2001-357861 (Patent Document 5). After a liquid seal is applied to groove portions corresponding to protruded portions of a polyelectrolytic membrane protruding from both electrodes, the polyelectrolytic membrane is held between a pair of separators so that temporary assembly is carried out, and the liquid seal is solidified in that state, so that a unit fuel cell is obtained. This technique is disclosed in Japanese Patent Application Laid-Open No. 2002-246044 (Patent Document 6). Further, a metal plate of a separator has a flow channel for fuel gas or oxidizing gas for fuel cell and flow channel for coolant, and a resin portion of the separator is has connecting holes for circulating fuel gas and oxidizing gas or coolant, whereby the separate for fuel cell is constituted. The separator is produced by integrally forming the metal plate and the resin portion by injection molding. This production technique is disclosed in Japanese Patent Application Laid-Open No. 2003-223903 (Patent Document 7).
Such fuel cells should realize excellent positioning or the like when the electric cell composed of MEA and the separator is formed or the electric cells are laminated in order to improve durability, airtightness and the like. In order to obtain high accuracy of positioning and the like, various techniques are proposed.
For example, the following fuel cell is disclosed in Japanese Patent Application Laid-Open No. 2000-12067 (Patent Document 8) as to the technique relating to positioning. In this fuel cell, a holding pin is inserted into a holding hole on a holding pin insertion side and a holding hole on a retaining ring insertion side so as to be combined with a retaining ring so as to hold an electric cell. A front edge of the holding pin is protruded from an outer surface of a separator and is fitted into a pin front edge insertion hole provided to one end of the holding pin of the adjacent electric cell so that the electric cells are laminated. The Patent Document 7 discloses the technique that improves corrosion resistance and airtightness of the fuel cell by the locating, and the Patent Document 2 discloses the joining technique for the resin layer that facilitates the joining between members and improves design effect by the locating.
In these conventional techniques, the metal mesh structure as the porous member to be used in the joining technique disclosed in the Patent Document 1 has a mesh form shown in FIG. 2. For this reason, in the case in which the resin is entwined with the metal mesh, the peel strength is heightened only in a specified direction (up-down and right-left directions in FIG. 2), and sufficient peel strength cannot be obtained in a residual direction (a direction vertical to a paper surface in FIG. 2). In recent years, therefore, the development of a method for joining the resin members and the porous members is demanded. The method is a method in which the peel strength is improved particularly in a thicknesswise direction of porous members so that the sufficient peel strength can be obtained in any directions, thereby sufficiently preventing peel in a specified direction.
The techniques disclosed in the Patent Documents 2 and 3, the same kind of materials are not directly joined, and laser absorptive resin should be used as a mating material of laser transmitting resin. The technique disclosed in the Patent Document 4 requires another member (solid inorganic matter) which absorbs a laser so as to generate heat when laser transmitting resins are joined. When, therefore, the techniques disclosed in the Patent Documents 2 to 4 are used so that resins are joined, plural kinds of members are required, and thus the production cost is high.
In the technique disclosed in the Patent Document 5, it is necessary to harden the liquid seal at a temperature and for a time within a range where components of MEA such as a solid polymer membrane and a catalytic electrode are not damaged. In the technique disclosed in the Patent Document 6, when differential pressure is applied to PEM that is present between a seal and a diffusion layer of the solid polymer electrolytic membrane exposed in the technique of the above Patent Document 5, excellent durability cannot be realized. Further, in the technique disclosed in the Patent Document 7, although the problem of the durability of the PEM can be solved, the separator is independent from the MEA, and excellent laminated assembly property cannot be realized. When a defective cell is replaced, a sealing position of another cell shifts, thereby deteriorating the airtightness of the cells.
In consideration of such situations, in recent years, in order to reduce the production cost, for example, the development of a technique of joining resin layers without members other than the laser transmitting resin is demanded. When the each unit of the fuel cell is joined, the development of a joining technique that enables satisfactory joining without complicatedly setting temperature and time at the time of thermosetting the resin is demanded. Further, the development of a method for joining thermoplastic resin and a fuel cell using this joining method is demanded. This method and fuel cell has a characteristic in which excellent durability is realized even when a differential pressure is applied to PEM and the excellent laminated assembly property is realized so that a shift of the sealing position of another cell is prevented at the time of replacing a defective cell, and excellent airtightness of the cells can be realized.
In addition, when the units of the fuel cell are laminated, a rubber seal or the like is generally used between the separators for holding the MEA. Since, however, dispersion of dimensional accuracy of the respective members in the units, dispersion of surface roughness and contact pressure of the seal surface, elution of defected impurity due to creep and hardening of materials, and the like are concerned, development of techniques that radically solve the problems of durability and productivity is demanded. The cells can be assembled by using the liquid seal, but since the assembly requires high-temperature calcination, this possibly exerts a harmful influence on the membrane and the catalyst of MEA. As a result, calcinating temperature and time are limited, and thus excellent productivity cannot be realized. Since the MEA is manually located in such cases, a shift occurs at the time of assembly, and various defects occur.
In the technique disclosed in the Patent Document 8, when the solid polymer fuel cell is assembled, a polymeric resin layer is hydrated into a saturation state, so that a PEM with low resistivity is formed. This excellent resistivity is, however, attenuated by drying of the membrane. In this technique, the electric cells are formed by holding MEA between the separators having seal, and after the drying of PEM is prevented at this stage, the electric cells are laminated so that the assembly of the fuel cell is formed. In this case, when the electric cells are formed, a structure in which a pair of the separators and the polymer membrane are pitted and they are fastened by pins or the like is adopted. For this reason, a pin fastening portion occupies a large area, and thus it is difficult to miniaturize the fuel cell.
In the technique disclosed in the Patent Document 7, in the structure of the separators for forming electric cells, an electrode corresponding portion is made of metal, a connecting hole for connecting a fluid is made of a resin layer, and a pin fastening portion for assembling the electric cells is omitted. For this reason, in this technique, the production process is simplified in comparison with the technique of the Patent Document 8. Since, however, the seal material is used, it is difficult to completely prevent the drying of the membrane.
The technique disclosed in the Patent Document 2 is the method for easily joining different kinds of the synthetic resin layer materials by laser without deteriorating the strength of the synthetic resin layer materials. This technical literature, however, does not describe that this technique can be applied to the fuel cell, and such a use is not examined.
In consideration of the situation, in recent years, the development of the following technique is demanded. In this technique, a pair of separators and the polymer membrane are not pitted, and they are joined without using a mechanical fastening unit such as pins, so that the fuel cell is miniaturized. Further, in order to realize excellent resistivity of the PEM, the development of a technique for preventing the drying of PEM is demanded. If the joining technique for different kinds of synthetic resin layer materials in the Patent Document 2 can be applied to the fuel cells, remarkable development of technology can be expected in the future in the field of fuel cells.